This invention relates to semi-permeable membranes substantially derived from bisphenol AF polycarbonates and polyestercarbonates. This invention further relates to the use of these membranes to separate gases.
The use of membranes to separate gases is well known in the art. Membranes have been used to recover or isolate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon dioxide, methane, and light hydrocarbons. Particular applications of interest include the separation of carbon dioxide from light hydrocarbons or other crude oil components as part of the tertiary oil recovery process. In other embodiments, nitrogen or helium is separated from natural gas. Other applications include the recovery of an enriched oxygen stream from air for use in enhanced combustion processes. Alternately, an enriched nitrogen stream may be obtained from air for use as an inert atmosphere over flammable fluids or for food storage.
Such membrane separations are based on the relative permeability of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than the other component(s) of the mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating component(s) and a stream which is depleted in the selectively permeating component(s). The stream which is depleted in the selectively permeating component(s) is enriched in the relatively nonpermeating component(s). A relatively nonpermeating component permeates more slowly through the membrane than the other component(s). An appropriate membrane material is chosen for the mixture so that some degree of separation of the gas mixture can be achieved.
Membranes for gas separation have been fabricated from a wide variety of polymeric materials, including cellulose triacetate; polyolefins such as polyethylene, polypropylene, and poly-4-methylpentene-1; and polysulfone. An ideal gas separation membrane is characterized by the ability to operate under high temperature and/or pressure while possessing a high separation factor (selectivity) and high gas permeability. The problem is finding membrane materials which possess all the desired characteristics. Polymers possessing high separation factors generally have low gas permeabilities, while those polymers possessing high gas permeabilities generally have low separation factors. In the past, a choice between a high separation factor and a high gas permeability has been unavoidably necessary. Furthermore, some of the membrane materials previously used have suffered from the disadvantage of poor performance under high operating temperatures and pressures. A membrane which possesses high selectivity, high gas permeability, and ability to operate under extreme conditions of temperature and pressure is needed.